COM (Computer Output Microfilm) systems using silver halide light-sensitive materials are working throughout the world as one of the systems of recording enormous information output from computers at high speed and in high density and storing it. Microfilms used for this system are called COM films and many kinds of such films are commercially available. COM films generally comprise a photographic support having provided on one side thereof one or more negative-working or direct positive silver halide photographic emulsion layers. These emulsions are generally designed to be of high micro-contrast so as to record a micro-image composed of a fine line copy with high resolving power. That is, they usually have a contrast of 1.5 to 2.5, though varying depending upon processing conditions. Aside from the COM use, information to be recorded on ordinary microfilms is obtained by directly photographing document copies, and hence most information is mainly micro-images composed of a fine line copy. However, a micro-image having a continuous gradation and uniform density must simultaneously be recorded with high quality. Although light-sensitive materials of a high macro-contrast designed to be optimal for recording a fine line copy can record a fine line copy with distinctness and good resolving power, they have the defect that, in recording macro-images, they are of so high a contrast that they naturally show a reduced ability to depict a shadow portion. As a means for overcoming this inconsistency, U.S. Pat. No. 4,924,773 and JP-A-55-33190 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example, disclose the technique of using light-sensitive materials in micro-recording which can be of a comparatively high contrast for micro-images and of a comparatively low contrast for macro-images. In this technique, it is proposed to use a previously surface-fogged silver halide emulsion together with an ordinary negative-working silver halide emulsion as a mixture. It is true that, in ordinary negative-working processing, a comparatively high micro-contrast and a comparatively low macro-contrast can be concurrently attained, but there is involved a fatal defect that, since the previously surface-fogged silver halide emulsion is developable independent of imagewise exposure, the minimum density (Dmin) of unexposed portions inevitably seriously increases. Thus, in the field of micro-light-sensitive materials in which a Dmin value of, preferably, 0.05 or less is required, such light-sensitive materials can never be put into practice.
On the other hand, in the field of microfilms for COM use, although information output on CRT of a computer is mostly of fine line copy and a high micro-contrast is required as is the same with micro-light-sensitive materials for ordinary documents, the demand for macro-contrast is not as severe as for micro-light-sensitive materials for documents. In addition, in COM use, it is required to provide a positive image as an original by reversal processing in view of the copying system's convenience after the processing. Therefore, a reversal developing process has long been employed. Processing steps of the generally well known reversal developing process comprise conducting a negative development using a black-and-white developer (generally called first developer) containing a silver halide solvent (usually NaSCN) and, after stopping the development, successively removing silver deposits in a negative image portion by silver-removing processing (generally using a bleaching solution such as a solution of potassium dichromate or cerium sulfate) without fixing processing, and successively conducting uniform exposure of the remaining, unexposed silver halide, to fog it, and again developing with a black-and-white developer (generally referred to as a second developer) to form a positive image. This reversal development manner is a manner having long been put into practice but, as has been set forth above, the steps are complicated and photographic properties (reversal Dmax, Dmin, sensitivity and gradation) are liable to be greatly changed depending upon processing conditions. These defects are described more specifically below. That is, since a considerably large quantity of a silver halide solvent (generally NaSCN being used in a concentration of 1 g to 6 g/liter) is present in the first developer of the reversal processing, dissolution of silver halide grains proceeds concurrently with development of silver halide grains, and the amount of remaining silver halide in unexposed portions after imagewise exposure is seriously influenced by processing conditions, composition, dilution ratio and fatigue degree of the first developer. Namely, photographic properties to be finally obtained, particularly reversal Dmax, directly depend upon the amount of remaining silver halide after the first development. In addition, photographic properties other than Dmax, particularly sensitivity and gradation, in this process are greatly influenced by the reversal Dmax. Hence, it has been eagerly desired in view of attaining stable processing to prepare a light-sensitive material which undergoes markedly slight or almost no changes in reversal Dmax even when types or formulations of processing solutions or the fatigue degree of the solutions are changed. In short, in view of the current market demand for microfilms used as COM which are adapted for high-speed, high-density recording of computer-output information, a light-sensitive material which can provide high-speed, high-quality micro-image properties for a short-time exposure of CRT and a high micro-contrast in, particularly, fine line recording and concurrently possesses excellent stability in processing and processing toughness in reversal accelerated processing has been desired.
In addition, in COM use, a high speed is required due to the peculiarity of the light source (CRT). However, conventional films for use as COM have insufficient sensitivity.
On the other hand, as a general technique for raising sensitivity of silver halide light-sensitive materials, there is a technique of enlarging the size of silver halide grains. However, enlarged silver halide grains provide a silver image having a decreased resolving power. This decrease in resolving power is a fatal defect in view of the use in microfilms.
Thus, appearance of high-speed microfilms with high resolving power for use as COM has eagerly been desired.